Aquametry or the determination of water is an important branch of analytical chemistry. Many analytical systems have been developed to determine water in solids, liquids and gases. Most of these systems are described in 3 volumes of Aquametry, Part I, II and III, J. Mitchell, Jr. and D. M. Smith; Wiley - Interscience; 1977, ISBN-0-471-02264-0 (Part I); 1984, ISBN-0-471-02265-9 (Part II); and 1980, ISBN-0-471-02266-7 (Part III).
Most determinations for water are easily made by a Karl Fischer titration. However, interferences are known including oxidizing agents, unsaturated compounds and thio compounds, see Aquametry, Part III, supra. Thermal conductivity detection gas chromatography (GC) is probably the second most used method often resulting in a water peak that elutes rapidly, e.g., 1 to 2 minutes, and with good sensitivity, e.g., 1 ppm, see Aquametry, Part I, supra. However, with GC the other components of a sample can take much longer to elute than water and can even decompose on-column and interfere with the analysis.
The present inventors were faced with the need to determine water in commercial formulations of dibromonitrilopropionamide (DBNPA), and antimicrobial product of The Dow Chemical Company. DBNPA is an oxidizing agent and reacts with iodide to yield iodine, and thus interferes with the Karl Fischer method. DBNPA is thermally labile and decomposed on-column in a GC. The products of the decomposition (believed to include HBr) corroded and eventually severed the filaments of the GC detector.
The present inventors thus considered high performance liquid chromatography (HPLC). Blasius et al. determined water by HPLC using a cyclic polyether column with a refractive index detector but water and other interfering components eluted without retention, Blasius et al., Talanta, 27:127, 1980. Fehrman et al. determined water by size-exclusion chromatography using a refractive index detector. Fehrman et al. used toluene as the eluent (rather than the more commonly used tetrahydrofuran) which significantly improved separation of water from other low molecular weight interfering components, Fehrman et al., Z. Fur Anal. Chem., 269(2):116, 1974. However, the DBNPA formulation was not miscible in toluene, and water itself has a limited solubility in toluene. Bjorkquist et al. reacted phenyl isocyanate with water to form N,N'diphenylurea (NN'DPU), with a total reaction time of about 1/2 hour, and then analyzed the NN'DPU by reverse phase HPLC, Bjorkquist et al., J. Chrom., 178:271, 1979. The present inventors wanted a simpler and faster procedure than this. Roof et al. used an anion-exchange column with a refractive index, ultraviolet absorption or differential density detector to determine water in a fluorination process stream in about 12 minutes, but with relatively poor column efficiency, i.e., about 30 effective theoretical plates and with poorer sensitivity than the present inventors desired, Roof et al., U.S. Pat. No. 3,935,097.
The determination of water without a prior separation by electrochemical means (for example by electrical conductivity measurement, dielectric constant measurement [dielometry] or oxidation/reduction reactions at electrodes) is extensively discussed in the volume titled Aquametry Part II, supra. However, such direct measurements can be seriously inaccurate due to variations in the sample composition unrelated to variations in water concentration. The foregoing patent and literature publications are fully incorporated herein by reference.
It is, accordingly, an objective of this invention to provide a liquid chromatographic system for the determination of water generally applicable but not limited to samples containing oxidizing agents, unsaturated compounds, thio compounds and thermally labile compounds, said system to be relatively rapid and accurate, said system to use an electrochemical detector.